1. Field of the Invention
The present invention relates to an image scanner apparatus for reading image information of an original set on an original read table line by line and outputting digital image data in units of lines and, more particularly, to an image scanner apparatus having improved read magnification precision in a main scanning direction.
2. Description of the Related Art
In a conventional image scanner apparatus for reading two-dimensional image information of an original and sending the read data to a host computer, an original placed on an original read table is generally irradiated with light from a light source, and light reflected by the original is focused on a line sensor by an optical lens. An image of the original set on the original read table is reduced or enlarged by the optical lens at a predetermined magnification, and a reduced or enlarged image is focused on the line sensor. A relationship (magnification) between a size of the original in its widthwise direction and a size of an image formed on the line sensor in its widthwise direction is set to satisfy a predetermined relationship (i.e., a defined magnification).
The defined magnification is obtained by appropriately adjusting a distance between the original read table and a carrier, a distance between the original read table and the optical lens, a distance between the optical lens and the line sensor, and mounting angles of the respective optical components. However, it is time-consuming and cumbersome to obtain an accurate magnification upon adjustment of the distances and angles described above.
The line sensor comprises 2,000 or more unit photoelectric elements (each corresponds to one pixel) in a line. It is impossible to cause the defined magnification to fall within an allowance of about several pixels by only adjustment of the mounting positions of the optical components described above. As a result, a total number of pixels (bits) of digital image data output from the image scanner apparatus to the host computer, in units of lines, does not necessarily correspond to the defined number of pixels set in correspondence to a sheet size such as a A4 or B5.